Mixed impure nickel/cobalt carbonates, basic carbonates, basic sulphates or hydroxides are often produced as intermediate products, as a step in the extraction of nickel and cobalt from their ores. These intermediates are then leached, selectively where possible to minimize impurities, and the resulting solutions treated in a variety of separation processes to produce refined nickel and refined cobalt. Leaching in ammoniacal solutions is well known and is in use commercially.
A process of treating a mixed nickel/cobalt hydroxide precipitate produced from pressure acid leaching of laterite ore is described by Simmons et al. in Canadian Patent no. 618826, where the mixed hydroxide precipitate can be releached in ammonium hydroxide, ammonium carbonate or ammonium sulphate alone or in combination.
The process developed for the Cawse Nickel project in Western Australia, was described in a paper presented at Nickel/Cobalt 97, Canada. The paper describes the process to extract nickel and cobalt from laterite ore by pressure acid leaching and describes the redissolution of the nickel and cobalt hydroxides produced as an intermediate by subjecting them to an ammonia leach, which it describes as “proven technology”.
The use of ammoniacal solutions for releaching the mixed nickel/cobalt hydroxide has the advantage that it is selective in preferentially extracting nickel and cobalt, leaving behind impurities such as iron, manganese, and magnesium. However it has the disadvantage that it does not extract all the nickel and cobalt from the hydroxides, leaving behind a residue that is rich in the two metals.
This does not constitute a major problem on a plant site where the ore is processed completely through to the final nickel and cobalt refined products in one location, as the nickel and cobalt rich residue from the releach step can be recycled to the initial ore treatment step to recover most of the metal values. An example of this has been used in commercial operation in Australia, where the high nickel and cobalt content residue from an inefficient leach of a mixed nickel/cobalt hydroxide in an ammonia ammonium carbonate solution was recycled to the primary ore leach stage to achieve an acceptable metal recovery efficiency.
However, mixed impure nickel/cobalt hydroxides, carbonates, basic carbonates or basic sulphates may be shipped as concentrated intermediates for processing at another site, without facilities for recycling the residue. The mixed nickel and cobalt hydroxides also exhibit a tendency to “age” subsequent to initial precipitation during the storage and transport process by processes associated with oxidation or recrystallisation, increasing the difficulty of dissolution, and increasing the amount of nickel and cobalt remaining in the leach residue. The incomplete dissolution of nickel and cobalt in the ammoniacal solution in these situations then necessitates a further expensive treatment step, or results in a loss of valuable product.
The use of reductants to enhance metal recovery in ammoniacal leaching systems is reported in the literature and various schemes have been proposed to recover metal values (principally nickel, cobalt and copper) from manganese oxide ocean nodules.
Steemson in a paper presented at the Alta Nickel/Cobalt Pressure Leaching and Hydrometallurgy Forum (1999) in Perth described the application of reductive leaching, using sulphur dioxide, for the recovery of nickel and cobalt from a mixed hydroxide in an ammonia/ammonium sulphate medium, as part of the Ramu project. In this process again, any unleached residue could be recycled to the ore leach stage for nickel and cobalt recovery.
The prior art processes do not provide for a complete and economical recovery of nickel and cobalt from a nickel, cobalt, or mixed nickel/cobalt hydroxide, carbonate, basic carbonate or basic sulphate material.
Furthermore the inventors have identified two different aging mechanisms operating when in particular, crude mixed nickel/cobalt hydroxides are stored for extended periods. The first aging process is oxidative and primarily affects cobalt redissolution and is largely overcome by the use of a reductant in the ammoniacal leach liquor. The second “aging” mechanism affects the nickel and appears to be associated with co-precipitation and/or recrystallisation processes within the solid.
An improvement on the prior art would therefore be to achieve a more complete dissolution and hence recovery of nickel and cobalt from a nickel, cobalt, or nickel/cobalt hydroxide, carbonate, basic carbonate, or basic sulphate material during the leaching process in an ammoniacal ammonium carbonate solution, overcoming any aging effects of the material, while minimizing releaching of impurities, and minimizing reagent costs. It is a desired feature of the present invention to provide a process that achieves improvements in the recovery of nickel and/or cobalt from nickel, cobalt, or mixed nickel/cobalt hydroxide, carbonate, basic carbonate or basic sulphate material.
The discussion of the background to the invention herein is included to explain the context of the invention. This is not to be taken as an admission that any of the material referred to was published, known or part of the common general knowledge in Australia as at the priority date of any of the claims.
Throughout the description and claims of the specification the word “comprise” and variations of the word, such as “comprising” and “comprises”, is not intended to exclude other additives, components, integers or steps.